The under-barrel grenade launcher, also known as an over-under, was a special forces weapon designed as a miniature rocket/grenade launcher. The weapon could be attached to a modified version of the DC-15A blaster rifle, DC-17m Interchangeable Weapon System, WESTAR-M5 blaster rifle or an E-15A blaster rifle via an underbarrel attachment, and could be configured to fire a variety of other explosive projectiles.
Advanced Recon Commandos put these weapons to use when rescuing Jedi Master Ki-Adi-Mundi and several other Jedi from General Grievous on Hypori. Although the salvo of grenades attacks did not significantly harm the incredibly agile Separatist general, they did manage to keep the cyborg preoccupied evading the explosive barrage.
Mace Windu also briefly used this weapon during his mission to Haruun Kal.
Voort saBinring used a blaster rifle with an integrated under-barrel grenade launcher on missions to Chashima (in 29 ABY) and Vandor-3 (in 44 ABY).
Specific examples of under-barrel grenade launchers include the Model MGL-1, the Viper & the Viper 2, and the HH-4.